I am not sure what you mean by weaker.

When you have a lack of Vacuum Issue or the Booster has an issue the Braking is harder because the Booster is not helping. When braking is harder on you then it does not brake as well.

The vacuum reservoir over the Fuel Tank is for the locking system and there is one or 2 check valves at the fire wall that are supposed to isolate that part of the system.

If you step on the brakes and the pedal sinks or you need to pump the brakes you still could be having issues with bleeding the brakes.

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84 300D, 82 Volvo 244Gl Diesel